Itinerary of high density lipoproteins in endothelial cells

Identification of Structures Labeled by Indocyanine Green in the Rat Choroid and Retina Can Guide Interpretation of Indocyanine Green Angiography

Purpose

Indocyanine green (ICG) is an albumin and lipoprotein binding dye absorbing in the far red used in angiography to visualize choroidal vessels (ICG angiography [ICGA]). To guide interpretation, ICG transport in the choroid, RPE, and retina of rats was studied.

Methods

Two conditions were used: RPE/choroid organoculture, incubated for 45 minutes in DMEM medium, 1% fetal bovine serum containing 0.25 mg/mL ICG and RPE/choroid and neural retina flat-mounts at 1 and 6 hours after intravenous ICG injection. Early and late sequences of ICGA were recorded until 6 hours. Ultra-deep red confocal microscope was used to localize ICG in flat-mounts and immunohistochemistry was performed for caveolin-1, tryptase (mast cell marker), and tubulin β3 (a nerve marker).

Results

In the organoculture, ICG penetrated homogeneously in the cytoplasm and stained the membranes of the RPE. At 1 hour after intravenous injection, ICG appeared in fine granules in RPE, partly labeled with caveolin-1 and decreasing at 6 hours. At 1 hour and 6 hours, ICG was found in the retinal vessels, faintly in the inner retina, and in the photoreceptor outer segments at 6 hours. In the choroid, ICG colocalized with mast cells, immunostained with tryptase, and accumulated along the large tubulin β3-labeled nerve bundles. The hypothesis was raised on the interpretation of late ICGA infrared photography in case of transthyretin amyloidosis with neuropathy.

Conclusions

Beside being a vascular dye, ICG is transported from the vessels to the RPE toward the outer retina. It stains mast cells and large choroidal nerves. These observations could help the analysis of ICGA images.

Boosting Cholesterol Efflux from Foam Cells by Sequential Administration of rHDL to Deliver MicroRNA and to Remove Cholesterol in a Triple-Cell 2D Atherosclerosis Model

Cardiovascular disease, the leading cause of mortality worldwide, is primarily caused by atherosclerosis, which is characterized by lipid and inflammatory cell accumulation in blood vessels and carotid intima thickening. Although disease management has improved significantly, new therapeutic strategies focused on accelerating atherosclerosis regression must be developed. Atherosclerosis models mimicking in vivo-like conditions provide essential information for research and new advances toward clinical application. New nanotechnology-based therapeutic opportunities have emerged with apoA-I nanoparticles (recombinant/reconstituted high-density lipoproteins, rHDL) as ideal carriers to deliver molecules and the discovery that microRNAs participate in atherosclerosis establishment and progression. Here, a therapeutic strategy to improve cholesterol efflux is developed based on a two-step administration of rHDL consisting of a first dose of antagomiR-33a-loaded rHDLs to induce adenosine triphosphate-binding cassette transporters A1 overexpression, followed by a second dose of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine rHDLs, which efficiently remove cholesterol from foam cells. A triple-cell 2D-atheroma plaque model reflecting the cellular complexity of atherosclerosis is used to improve efficiency of the nanoparticles in promoting cholesterol efflux. The results show that sequential administration of rHDL potentiates cholesterol efflux indicating that this approach may be used in vivo to more efficiently target atherosclerotic lesions and improve prognosis of the disease.

Understanding Myeloperoxidase-Induced Damage to HDL Structure and Function in the Vessel Wall: Implications for HDL-Based Therapies

Atherosclerosis is a disease of increased oxidative stress characterized by protein and lipid modifications in the vessel wall. One important oxidative pathway involves reactive intermediates generated by myeloperoxidase (MPO), an enzyme present mainly in neutrophils and monocytes. Tandem MS analysis identified MPO as a component of lesion derived high-density lipoprotein (HDL), showing that the two interact in the arterial wall. MPO modifies apolipoprotein A1 (apoA-I), paraoxonase 1 and certain HDL-associated phospholipids in human atheroma. HDL isolated from atherosclerotic plaques depicts extensive MPO mediated posttranslational modifications, including oxidation of tryptophan, tyrosine and methionine residues, and carbamylation of lysine residues. In addition, HDL associated plasmalogens are targeted by MPO, generating 2-chlorohexadecanal, a pro-inflammatory and endothelial barrier disrupting lipid that suppresses endothelial nitric oxide formation. Lesion derived HDL is predominantly lipid-depleted and cross-linked and exhibits a nearly 90% reduction in lecithin-cholesterol acyltransferase activity and cholesterol efflux capacity. Here we provide a current update of the pathophysiological consequences of MPO-induced changes in the structure and function of HDL and discuss possible therapeutic implications and options. Preclinical studies with a fully functional apoA-I variant with pronounced resistance to oxidative inactivation by MPO-generated oxidants are currently ongoing. Understanding the relationships between pathophysiological processes that affect the molecular composition and function of HDL and associated diseases is central to the future use of HDL in diagnostics, therapy, and ultimately disease management.

Apolipoprotein M and Sphingosine-1-Phosphate Receptor 1 Promote the Transendothelial Transport of High-Density Lipoprotein

Objective: ApoM enriches S1P (sphingosine-1-phosphate) within HDL (high-density lipoproteins) and facilitates the activation of the S1P1 (S1P receptor type 1) by S1P, thereby preserving endothelial barrier function. Many protective functions exerted by HDL in extravascular tissues raise the question of how S1P regulates transendothelial HDL transport. Approach and Results: HDL were isolated from plasma of wild-type mice, Apom knockout mice, human apoM transgenic mice or humans and radioiodinated to trace its binding, association, and transport by bovine or human aortic endothelial cells. We also compared the transport of fluorescently-labeled HDL or Evans Blue, which labels albumin, from the tail vein into the peritoneal cavity of apoE-haploinsufficient mice with (apoE-haploinsufficient mice with endothelium-specific knockin of S1P1) or without (control mice, ie, apoE-haploinsufficient mice without endothelium-specific knockin of S1P1) endothelium-specific knockin of S1P1. The binding, association, and transport of HDL from Apom knockout mice and human apoM-depleted HDL by bovine aortic endothelial cells was significantly lower than that of HDL from wild-type mice and human apoM-containing HDL, respectively. The binding, uptake, and transport of 125I-HDL by human aortic endothelial cells was increased by an S1P1 agonist but decreased by an S1P1 inhibitor. Silencing of SR-BI (scavenger receptor BI) abrogated the stimulation of 125I-HDL transport by the S1P1 agonist. Compared with control mice, that is, apoE-haploinsufficient mice without endothelium-specific knockin of S1P1, apoE-haploinsufficient mice with endothelium-specific knockin of S1P1 showed decreased transport of Evans Blue but increased transport of HDL from blood into the peritoneal cavity and SR-BI expression in the aortal endothelium. Conclusions: ApoM and S1P1 promote transendothelial HDL transport. Their opposite effect on transendothelial transport of albumin and HDL indicates that HDL passes endothelial barriers by specific mechanisms rather than passive filtration.

Plasma HDL cholesterol and risk of dementia - observational and genetic studies

AIMS

The association of plasma high-density lipoprotein (HDL) cholesterol with risk of dementia is unclear. We therefore tested the hypothesis that high levels of plasma HDL cholesterol are associated with increased risk of dementia and whether a potential association is of a causal nature.

METHODS AND RESULTS

In two prospective population-based studies, the Copenhagen General Population Study and the Copenhagen City Heart Study (N = 111,984 individuals), we first tested whether high plasma HDL cholesterol is associated with increased risk of any dementia and its subtypes. These analyses in men and women separately were adjusted multifactorially for other risk factors including apolipoprotein E (APOE) genotype. Second, taking advantage of two-sample Mendelian randomization, we tested whether genetically elevated HDL cholesterol was causally associated with Alzheimer's disease using publicly available consortia data on 643,836 individuals. Observationally, multifactorially adjusted Cox regression restricted cubic spline models showed that both men and women with extreme high HDL cholesterol concentrations had increased risk of any dementia and of Alzheimer's disease. Men in the 96th-99th and 100th versus the 41st-60th percentiles of HDL cholesterol had multifactorially including APOE genotype adjusted hazard ratios of 1.66 (95% confidence interval 1.30-2.11) and 2.00 (1.35-2.98) for any dementia and 1.59 (1.16-2.20) and 1.87 (1.11-3.16) for Alzheimer's disease. Corresponding estimates for women were 0.94 (0.74-1.18) and 1.45 (1.03-2.05) for any dementia and 0.94 (0.70-1.26) and 1.69 (1.13-2.53) for Alzheimer's disease. Genetically, the two-sample Mendelian randomization odds ratio for Alzheimer's disease per 1 standard deviation increase in HDL cholesterol was 0.92 (0.74-1.10) in the IGAP2019 consortium and 0.98 (0.95-1.00) in the ADSP/IGAP/PGC-ALZ/UKB consortium. Similar estimates were observed in sex stratified analyses.

CONCLUSION

High plasma HDL cholesterol was observationally associated with increased risk of any dementia and Alzheimer's disease, suggesting that HDL cholesterol can be used as an easily accessible plasma biomarker for individual risk assessment.

TRANSLATIONAL PERSPECTIVE

The present study identifies very high plasma HDL cholesterol levels as an independent risk factor for any dementia and Alzheimer's disease in both men and women of the general population. Two-sample Mendelian randomization studies do not support that this association is of a causal nature, indicating HDL cholesterol as a non-causal risk factor for Alzheimer's disease. Our findings suggest that very high HDL cholesterol can be used as an easily accessible plasma biomarker to evaluate increased risk of dementia and potential identification of high-risk individuals for early targeted prevention - an area highly recommended to direct attention towards.

The Endothelium Is Both a Target and a Barrier of HDL's Protective Functions

The vascular endothelium serves as a barrier between the intravascular and extravascular compartments. High-density lipoproteins (HDL) have two kinds of interactions with this barrier. First, bloodborne HDL must pass the endothelium to access extravascular tissues, for example the arterial wall or the brain, to mediate cholesterol efflux from macrophages and other cells or exert other functions. To complete reverse cholesterol transport, HDL must even pass the endothelium a second time to re-enter circulation via the lymphatics. Transendothelial HDL transport is a regulated process involving scavenger receptor SR-BI, endothelial lipase, and ATP binding cassette transporters A1 and G1. Second, HDL helps to maintain the integrity of the endothelial barrier by (i) promoting junction closure as well as (ii) repair by stimulating the proliferation and migration of endothelial cells and their progenitor cells, and by preventing (iii) loss of glycocalix, (iv) apoptosis, as well as (v) transmigration of inflammatory cells. Additional vasoprotective functions of HDL include (vi) the induction of nitric oxide (NO) production and (vii) the inhibition of reactive oxygen species (ROS) production. These vasoprotective functions are exerted by the interactions of HDL particles with SR-BI as well as specific agonists carried by HDL, notably sphingosine-1-phophate (S1P), with their specific cellular counterparts, e.g., S1P receptors. Various diseases modify the protein and lipid composition and thereby the endothelial functionality of HDL. Thorough understanding of the structure-function relationships underlying the multiple interactions of HDL with endothelial cells is expected to elucidate new targets and strategies for the treatment or prevention of various diseases.

Transendothelial transport of lipoproteins

The accumulation of low-density lipoproteins (LDL) in the arterial wall plays a pivotal role in the initiation and pathogenesis of atherosclerosis. Conversely, the removal of cholesterol from the intima by cholesterol efflux to high density lipoproteins (HDL) and subsequent reverse cholesterol transport shall confer protection against atherosclerosis. To reach the subendothelial space, both LDL and HDL must cross the intact endothelium. Traditionally, this transit is explained by passive filtration. This dogma has been challenged by the identification of several rate-limiting factors namely scavenger receptor SR-BI, activin like kinase 1, and caveolin-1 for LDL as well as SR-BI, ATP binding cassette transporter G1, and endothelial lipase for HDL. In addition, estradiol, vascular endothelial growth factor, interleukins 6 and 17, purinergic signals, and sphingosine-1-phosphate were found to regulate transendothelial transport of either LDL or HDL. Thorough understanding of transendothelial lipoprotein transport is expected to elucidate new therapeutic targets for the treatment or prevention of atherosclerotic cardiovascular disease and the development of strategies for the local delivery of drugs or diagnostic tracers into diseased tissues including atherosclerotic lesions.

Microencapsulated Pomegranate Modifies the Composition and Function of High-Density Lipoproteins (HDL) in New Zealand Rabbits

Previous studies demonstrated that pomegranate, which is a source of several bioactive molecules, induces modifications of high-density lipoproteins (HDL) lipid composition and functionality. However, it remains unclear whether the beneficial effects of pomegranate are related to improvement in the lipid components of HDL. Therefore, in this placebo-controlled study, we characterized the size and lipid composition of HDL subclasses and assessed the functionality of these lipoproteins after 30 days of supplementation with a pomegranate microencapsulated (MiPo) in New Zealand white rabbits. We observed a significant decrease in plasma cholesterol, triglycerides, and non-HDL sphingomyelin, as well as increases in HDL cholesterol and HDL phospholipids after supplementation with MiPo. Concomitantly, the triglycerides of the five HDL subclasses isolated by electrophoresis significantly decreased, whereas phospholipids, cholesterol, and sphingomyelin of HDL subclasses, as well as the HDL size distribution remained unchanged. Of particular interest, the triglycerides content of HDL, estimated by the triglycerides-to-phospholipids ratio, decreased significantly after MiPo supplementation. The modification on the lipid content after the supplementation was associated with an increased resistance of HDL to oxidation as determined by the conjugated dienes formation catalyzed by Cu²⁺. Accordingly, paraoxonase-1 (PON1) activity determined with phenylacetate as substrate increased after MiPo. The effect of HDL on endothelial function was analyzed by the response to increasing doses of acetylcholine of aorta rings co-incubated with the lipoproteins in an isolated organ bath. The HDL from rabbits that received placebo partially inhibited the endothelium-dependent vasodilation. In contrast, the negative effect of HDL on endothelial function was reverted by MiPo supplementation. These results show that the beneficial effects of pomegranate are mediated at least in part by improving the functionality of HDL, probably via the reduction of the content of triglycerides in these lipoproteins.

The uptake mechanism and intracellular fate of Paraoxonase-1 in endothelial cells

Paraoxonase-1 (PON1) is a high-density lipoprotein (HDL)-associated lactonase that plays a significant role in the anti-atherosclerotic activity of HDL. However, several studies have shown that PON1 localizes in cells, where it operates independently of HDL. Previously, we showed that PON1 localizes in endothelial cells (ECs), and impairs vasodilation mediated by the endothelium-derived hyperpolarizing factor (EDHF) 5,6-δ-DHTL. However, the internalization pathway of PON1 into ECs, and the intracellular fate of PON1 are unknown. Therefore, the present study aimed to elucidate the uptake mechanism, intracellular trafficking and the function of PON1 in ECs. ‏We conducted a series of inhibition experiments of fluorescently labeled recombinant PON1 (rePON1) in ECs, followed by FACS analyses. We found that rePON1 binds the EC membrane via specific binding sites located in lipid-rafts/caveolae microdomains that are shared with HDL, and internalized through dynamin-dependent endocytosis. Qualitative assessments of the intracellular trafficking of rePON1, using confocal z-stack images, showed colocalization of the labeled rePON1 with early and late endosome/lysosome markers. Accordingly, a "pulse-chase" incubation of rePON1, followed by lactonase activity measurement in EC lysate, revealed that rePON1 retains its lactonase activity after binding to the cells. However, this activity decreases over time. Finally, induction of endothelial dysfunction with high glucose, angiotensin II, or palmitic acid increased rePON1 uptake by ECs. In conclusion, these results indicate that free PON1 interacts with ECs via binding sites located in lipid-rafts/caveolae, where it is enzymatically active and regulates endothelial functions. However, once internalized, PON1 is degraded. Additionally, alteration in endothelial function affects PON1 uptake by ECs.

[Metabolism and Function of High-Density Lipoproteins (HDL)]

Metabolism and Function of High-Density Lipoproteins (HDL) Abstract. HDL has long been considered as 'good cholesterol', beneficial to the whole body and in particular to cardio-vascular health. However, HDL is a complex particle that undergoes dynamic remodeling through interactions with various enzymes and tissue types throughout its life cycle. In this review, we explore the novel understanding of HDL as a multifaceted class of lipoprotein, with multiple subclasses of different size, molecular composition, receptor interactions, and functionality, in health and disease. Further, we report on emergent HDL based therapeutics tested in small and larger scale clinical trials and their mixed successes.

HDL-Mediated Lipid Influx to Endothelial Cells Contributes to Regulating Intercellular Adhesion Molecule (ICAM)-1 Expression and eNOS Phosphorylation

Reverse cholesterol transport (RCT) is considered as the most important antiatherogenic role of high-density lipoproteins (HDL), but interventions based on RCT have failed to reduce the risk of coronary heart disease. In contrast to RCT, important evidence suggests that HDL deliver lipids to peripheral cells. Therefore, in this paper, we investigated whether HDL could improve endothelial function by delivering lipids to the cells. Internalization kinetics using cholesterol and apolipoprotein (apo) AI fluorescent double-labeled reconstituted HDL (rHDL), and human dermal microvascular endothelial cells-1 (HMEC-1) showed a fast cholesterol influx (10 min) and a slower HDL protein internalization as determined by confocal microscopy and flow cytometry. Sphingomyelin kinetics overlapped that of apo AI, indicating that only cholesterol became dissociated from rHDL during internalization. rHDL apo AI internalization was scavenger receptor class B type I (SR-BI)-dependent, whereas HDL cholesterol influx was independent of SR-BI and was not completely inhibited by the presence of low-density lipoproteins (LDL). HDL sphingomyelin was fundamental for intercellular adhesion molecule-1 (ICAM-1) downregulation in HMEC-1. However, vascular cell adhesion protein-1 (VCAM-1) was not inhibited by rHDL, suggesting that components such as apolipoproteins other than apo AI participate in HDL's regulation of this adhesion molecule. rHDL also induced endothelial nitric oxide synthase eNOS S1177 phosphorylation in HMEC-1 but only when the particle contained sphingomyelin. In conclusion, the internalization of HDL implies the dissociation of lipoprotein components and a SR-BI-independent fast delivery of cholesterol to endothelial cells. HDL internalization had functional implications that were mainly dependent on sphingomyelin. These results suggest a new role of HDL as lipid vectors to the cells, which could be congruent with the antiatherogenic properties of these lipoproteins.

Endothelial Transcytosis of Lipoproteins in Atherosclerosis

Seminal studies from Nikolai Anichckov identified the accumulation of cholesterol in the arteries as the initial event that lead to the formation of atherosclerotic plaques. Further studies by Gofman and colleagues demonstrated that high levels of circulating low-density lipoprotein cholesterol (LDL-C) was responsible for the accelerated atherosclerosis observed in humans. These findings were confirmed by numerous epidemiological studies which identified elevated LDL-C levels as a major risk factor for cardiovascular disease. LDL infiltrates in the arterial wall and interacts with the proteoglycan matrix promoting the retention and modification of LDL to a toxic form, which results in endothelial cell (EC) activation and vascular inflammation. Despite the relevance of LDL transport across the endothelium during atherogenesis, the molecular mechanism that control this process is still not fully understood. A number of studies have recently demonstrated that low density lipoprotein (LDL) transcytosis across the endothelium is dependent on the function of caveolae, scavenger receptor B1 (SR-B1), activin receptor-like kinase 1 (ALK1), and LDL receptor (LDLR), whereas high-density lipoproteins (HDL) and its major protein component apolipoprotein AI transcytose ECs through SR-B1, ATP-Binding cassette transporter A1 (ABCA1) and ABCG1. In this review article, we briefly summarize the function of the EC barrier in regulating lipoprotein transport, and its relevance during the progression of atherosclerosis. A better understanding of the mechanisms that mediate lipoprotein transcytosis across ECs will help to develop therapies targeting the early events of atherosclerosis and thus exert potential benefits for treating atherosclerotic vascular disease.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

Characterization of immortalized human dermal microvascular endothelial cells (HMEC-1) for the study of HDL functionality

BACKGROUND

Primary cultures endothelial cells have been used as models of endothelial related diseases such atherosclerosis. Biological behavior of primary cultures is donor-dependent and data could not be easily reproducible; endothelial cell lines are emerging options, particularly, human dermal microvascular endothelial cells (HMEC-1), that should be validated to substitute primary cultures for the study of HDL functions.

METHODS

Morphology, size and granularity of cells were assessed by phase contrast microscopy and flow cytometry of HMEC-1. The adhesion molecules, ICAM-1and VCAM-1 after TNF-α stimulation, and endothelial markers CD105 endoglin, as well as HDL receptor SR-BI were determined by flow cytometry. Internalization of HDL protein was demonstrated by confocal microscopy using HDL labeled with Alexa Fluor 488. HUVECs were used as reference to compared the characteristics with HMEC-1.

RESULTS

HMEC-1 and HUVEC had similar morphologies, size and granularity. HMEC-1 expressed endothelial markers as HUVECs, as well as functional SR-B1 receptor since the cell line was able to internalize HDL particles. HMEC-1 effectively increased ICAM-1 and VCAM-1 expression after TNF-α stimulation. HUVECs showed more sensibility to TNF-α stimulus but the range of ICAM-1 and VCAM-1 expression was less homogeneous than in HMEC-1, probably due to biological variation of the former. Finally, the expression of adhesion molecules in HMEC-1 was attenuated by co-incubation with HDL.

CONCLUSION

HMEC-1 possess characteristics of endothelial cells, similar to HUVECs, being a cell line suitable to evaluate the functionality of HDL vis-à-vis the endothelium.

Novel mechanism of regulation of the 5-lipoxygenase/leukotriene B4 pathway by high-density lipoprotein in macrophages

High-density lipoprotein (HDL) interacts with various cells, particularly macrophages, in functional cell-HDL interactions. Here, we found that HDL protein quality and lipid quality play critical roles in HDL functions. HDL fractions from healthy volunteers (HDL_Healthy) and patients with recurrent coronary atherosclerotic disease (HDL_CAD) were prepared. To analyse functional HDL-macrophage interactions, macrophages were co-incubated with each HDL, and lipid mediator production was assessed by liquid chromatography/mass spectrometry-based metabololipidomics. HDL_Healthy treatment attenuated the pro-inflammatory lipid mediator production, particularly that of leukotriene (LT) B₄, and this treatment enhanced lipoxin (LX) B₄ and resolvin (Rv) E2 production. HDL_Healthy treatment enhanced the proteasome-mediated degradation of the LTB₄-producing enzyme 5-lipoxygenase (LO) in activated macrophages; however, HDL_CAD did not show these anti-inflammatory effects. HDL_Healthy was engulfed by macrophages via clathrin-mediated endocytosis, which was a critical step in 5-LO/LTB₄ regulation. We also found that HDL_CAD showed higher levels of the LTB₄-producing enzymes and thus promoted LTB₄ production from HDL_CAD. In addition, LTB₄ attenuated HDL endocytosis, HDL-mediated 5-LO degradation in macrophages, and HDL-derived augmentation of macrophage phagocytosis. These results indicated that local LTB₄ produced de novo from HDL_CAD regulates HDL-macrophage functional interactions and plays critical roles in dysfunctional, inflammatory HDL characteristics.

VEGF-A Regulates Cellular Localization of SR-BI as Well as Transendothelial Transport of HDL but Not LDL

Objective—

Low- and high-density lipoproteins (LDL and HDL) must pass the endothelial layer to exert pro- and antiatherogenic activities, respectively, within the vascular wall. However, the rate-limiting factors that mediate transendothelial transport of lipoproteins are yet little known. Therefore, we performed a high-throughput screen with kinase drug inhibitors to identify modulators of transendothelial LDL and HDL transport.

Approach and Results—

Microscopy-based high-content screening was performed by incubating human aortic endothelial cells with 141 kinase-inhibiting drugs and fluorescent-labeled LDL or HDL. Inhibitors of vascular endothelial growth factor (VEGF) receptors (VEGFR) significantly decreased the uptake of HDL but not LDL. Silencing of VEGF receptor 2 significantly decreased cellular binding, association, and transendothelial transport of 125 I-HDL but not 125 I-LDL. RNA interference with VEGF receptor 1 or VEGF receptor 3 had no effect. Binding, uptake, and transport of HDL but not LDL were strongly reduced in the absence of VEGF-A from the cell culture medium and were restored by the addition of VEGF-A. The restoring effect of VEGF-A on endothelial binding, uptake, and transport of HDL was abrogated by pharmacological inhibition of phosphatidyl-inositol 3 kinase/protein kinase B or p38 mitogen-activated protein kinase, as well as silencing of scavenger receptor BI. Moreover, the presence of VEGF-A was found to be a prerequisite for the localization of scavenger receptor BI in the plasma membrane of endothelial cells.

Conclusions—

The identification of VEGF as a regulatory factor of transendothelial transport of HDL but not LDL supports the concept that the endothelium is a specific and, hence, druggable barrier for the entry of lipoproteins into the vascular wall.

HDLs in crises

PURPOSE OF REVIEW

The clinical utility of HDLs has been scrutinized upon the publication of Mendelian randomization studies showing no effect of HDL-cholesterol (HDL-C) modifying variants on cardiovascular disease (CVD) outcome. The failures of randomized controlled HDL-C-directed intervention trials have further fueled this skepticism. This general criticism originates from oversimplification that has equated 'HDL-C' with 'HDL' and misconceived both as the 'good cholesterol'.

RECENT FINDINGS

HDL particles are heterogeneous and carry hundreds of different lipids, proteins, and microRNAs. Many of them but not cholesterol, that is, HDL-C, contributes to the multiple protective functions of HDLs that probably evolved to manage potentially life-threatening crises. Inflammatory processes modify the composition of HDL particles as well as their individual protein and lipid components, and, as a consequence, also their functionality. Gain of dominant-negative functions makes dysfunctional HDL a part rather than a solution of the endangering situation. Quantification of HDL particle numbers, distinct proteins or lipids, and modifications thereof as well as bioassays of HDL functionality are currently explored toward their diagnostic performance in risk prediction and monitoring of treatment response.

SUMMARY

Any successful clinical exploitation of HDLs will depend on the identification of the most relevant (dys)functions and their structural correlates. Stringent or prioritized structure-(dys)function relationships may provide biomarkers for better risk assessment and monitoring of treatment response. The most relevant agonists carried by either functional or dysfunctional HDLs as well as their cellular responders are interesting targets for drug development.